Sunday, 22 April 2012

Anomalous Expansion Of Water

DEFINITION

Water shows unusual expansion.When it is cooled from four degree centigrade to
zero degree centigrade is known as "anomalous expansion of water."The unusual behaviour of water, when it expands below4° celsius to 0° is called anmalus expansion of water.

EFFECTS
The anomalous expansion of water helps preserve aquatic life during very
cold weather. When temperature falls, the top layer of water in a pond
contracts, becomes denser and sinks to the bottom. A circulation is thus
set up until the entire water in the pond reaches its maximum density
at 4°C. If the temperature falls further, the top layer expands and
remains on the top till it freezes. Thus even though the upper layer are
frozen the water near the bottom is at 4°C and the fishes etc. can
survive in it easily.

SCINTIFIC REASON

Anomalous expansion of water takes place because when water is heated to
277K hydrogen bonds are formed. Though ice is supposed to expand when
it is converted into water, this gradual formation of hydrogen bonds
causes it to contract, i.e. the contraction caused due to the formation
of hydrogen bonds is greater than the actual expansion of ice. At 277K
water has the maximum density because all the hydrogen bonds are formed
by 277K beyond which water obeys the kinetic theory of molecules, an
increase in volume when heated and the reverse when cooled. The same
thing happens in the reverse when water is cooled beyond 277K.

EXAMPLE

If we take a cube of ice at -5°C and heat
it, it expands till ice starts melting. During melting its temperature
remains 0°C but its volume decreases. If heat is continuously supplied
to water at 0°C, it further contracts up to 4°C and then it starts
expanding. Thus water has its minimum volume and maximum density at 4°C.

Water is different. With most everything on Earth except freshwater, the colder it gets, the more dense it becomes. Take alcohol for instance. If we were to fill up a 1 liter container with pure alcohol at 30 degrees Celsius (86 Fahrenheit) and then take another 1 liter container and fill it with pure alcohol at 10 degrees Celsius (50 Fahrenheit) the cooler container of alcohol would weigh more. This is because the cooler alcohol is more dense, so more alcohol molecules can fit in the same container. This is true with freshwater too, up to a point. But at about 4 degrees Celsius (40 Fahrenheit) water reaches its densest point. Amazingly, as water cools further, it actually becomes less dense.

Each water molecule is made of 2 hydrogen atoms and 1 oxygen atom. These are connected to one another by very strong chemical bonds called covalent bonds. Water molecules are connected to each other by much weaker chemical bonds called hydrogen bonds between the positively charged hydrogen atoms, and one negatively charged oxygen atom in a neighboring water molecule.

As water gets colder than 4 degrees Celsius (40 Fahrenheit) the hydrogen bonds connecting different water molecules adjust to keep the negatively charged oxygen atoms apart. This results in a crystal latice which begins to form at less than 4 degrees Celsius. This crystal latice is completely formed at freezing, and is commonly known as ice.

So, why does ice float? Like most things that float, ice floats because it is less dense than liquid water. Ice is about 9% less dense. When ice forms, it takes up about 9% more space than it did as a liquid. Thus, a 1 liter container of ice weighs less than a 1 liter container of liquid water, and the lighter material floats to the top. As we said, water is different............... 0:-)

Thank you .... that was very informative.... may be stupid question but, why not water losing temperature under the ice and becoming ice. it is in contact with ice right and ice is at 0 degree Celsius, so why not ice blanket covering water is not increasing...